For this definition every point on a circle is an extreme point so it makes sense to say it has infinitely (uncountably!) many corners. Though the notion of side is not as good. If the definition is line segment joining two vertices then the answer would be 0 for the circle.
Just so, are circles and ovals the same?
Circles and ovals are both types of ellipses. An 'oval' is really the informal term for an 'ellipse', whereas a 'circle' is an ellipse where the semi-major and semi-minor axes are equal. If you're talking about higher-dimensions, the word you are looking for is probably ellipsoid. (A sphere is an example.)
Can an oval have straight sides?
An oval has no straight sides and no corners, but it does have 1 face, which can be traced or seen as a flat shape when looking at it.